Interpretive Summary: Conversion of plant cell walls to ethanol is a multi-step process (pretreatment, enzymatic hydrolysis, and fermentation) in which each stage presents a multitude of challenges. This paper describes the key hurdles that must be surpassed and presents new data that illustrates some of the various problems.

Technical Abstract:
Conversion of plant cell walls to ethanol constitutes generation 2 bioethanol production. The process consists of several steps: biomass selection/genetic modification, physiochemical pretreatment, enzymatic saccharification, fermentation, and separation. Ultimately, it is desired to combine as many of the biochemical steps as possible in a single organism to achieve consolidated bioprocessing (CBP). A commercially-ready CBP organism is currently unreported. Production of generation 2 bioethanol is hindered by economics, particularly in the cost of pretreatment (including waste management and solvent recovery), the cost of saccharification enzymes (particularly exocellulases and endocellulases displaying kcat ~1 s-1 on crystalline cellulose), and the inefficiency of cofermentation of five and six carbon monosaccharides (owing in part to redox cofactor imbalances in Saccharomyces cerevisiae).